Variables and Types

Rust has a rich type system designed for safety and expressiveness. Variables are immutable by default, and the compiler infers types whenever possible while enforcing strict type safety.

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Variables and Mutability

Immutable by Default

In Rust, variables are immutable by default:

let x = 5;
x = 6; // ERROR: cannot assign twice to immutable variable

To make a variable mutable, use the mut keyword:

let mut x = 5;
x = 6; // OK

Tip: Preferring immutability makes code easier to reason about. Only use mut when you actually need to change a value.

Constants

Constants are always immutable and must have a type annotation:

const MAX_POINTS: u32 = 100_000;

Feature	let	const
Mutability	Can be mut	Always immutable
Type annotation	Optional (inferred)	Required
Computed at	Runtime	Compile time
Scope	Block scope	Any scope (including global)

Shadowing

Rust allows you to declare a new variable with the same name, shadowing the previous one: